Optical fiber communication systems typically are based on amplitude modulation (AM) and direct detection of the optical energy. Optical fiber communication systems based on frequency modulation (FM) may offer performance benefits. However, satisfactory frequency tunable lasers, needed as transmitters in such systems, are not generally available. It has been found that two-cavity resonator laser structures provide optical amplification and direct frequency modulation. Such optically coupled semiconductor diode lasers are therefore of interest in optical fiber FM communication systems. The optical coupling is produced when two semiconductor diode laser cavities, i.e. Fabry-Perot cavities, are disposed end-to-end in a special way. The two semiconductor diode laser cavities must be very precisely longitudinally aligned and very precisely spaced apart only a very small distance, in order to produce a satisfactory optical coupling. In such instance, modulation of injection current for a smaller one of two semiconductor lasers produces a frequency shift in the output of the larger laser, which constitutes optical amplification.
Such structures appear to be quite useful in frequency modulated optical communication systems. However, an important factor limiting their use is the complexity involved in fabricating them. For example, it is well known that higher diode laser efficiency is obtained if the laser's Fabry-Perot cavity is more rectangular. By that I mean that the various faces of semiconductor bodies forming the laser cavities are more parallel or orthogonal to one another, as are produced by cleaving along crystal planes. On the other hand, obtaining precise alignment and precise close spacing of a pair of such cleaved bodies is difficult. If not precisely aligned and spaced, optimum optical coupling, that is frequency modulation efficiency, will not be obtained. Alignment, and to some extent spacing, problems can be avoided by making both laser cavities in a single body. This is achieved by etching a groove in one face of an elongated body, so as to divide the body into two integral Fabry-Perot cavities on that face. On the other hand, normal etching techniques do not produce the desired flat and parallel facing ends on the two resulting integral Fabry-Perot cavities. Thus, high efficiency laser cavities are not obtained, which in turn reduces frequency modulation efficiency.
I have found a simple method for producing both high efficiency laser cavities and high optical coupling. In a sense, the best of both the prior techniques is provided. In my method, I use well-established cleaving techniques to form high efficiency laser cavities. Cleaved semiconductor diode laser bodies are automatically highly longitudinally aligned and then precisely spaced, to form a very closely coupled two-cavity resonator. Concurrently, the laser bodies are bonded in place, so that their close optical coupling is preserved. In substance, I have therefore found an improved technique for producing cleaved-coupled-cavity (C.sup.3) semiconductor lasers.